In a sheet-fed offset printing machine, a printing plate is fastened on a plate cylinder by means of a clamping rail allocated to the leading edge of the printing plate and a clamping rail allocated to the trailing edge. An example of such a printing machine may be seen in U.S. Pat. No. 5,511,478 (DE 43 39 344 C1) which discloses a device for the automatic changing of printing plates in a printing machine. Here, a printing plate, which is to be inserted, has certain built in guards to stop movement and a guide device to assist its contact with the cylinder and insertion into the clamping rail of the edges. After this plate has been inserted into the clamping rail and the plate's leading edge has been clamped, the plate is drawn onto the cylinder by the latter being rotated forwards. During insertion into the leading edge clamping rail and the drawing process, the central or rear part of the printing plate still makes contact with the guard.
To assure high print quality, printing plates are usually fitted to the plate cylinder in a precise location using a register system. For example, U.S. Pat. No. 5,383,402 (EP 0 551 976 A1) discloses a register system for mounting a plate on a plate cylinder that includes a plate lockup device, reference pins and a lamp. The plate lockup device is provided in a gap formed in the circumference surface of the plate cylinder. The reference pins are electrically rendered conductive by contacting an insertion end of a plate inserted into the plate lockup device, thereby detecting insertion of the plate. The lamp confirms and indicates insertion of the plate from an output from the reference pins thereby allowing a manual installation of a printing plate to visually verified.
In the case of automatic printing plate changers, the signal from register interrogation is evaluated by a control system, so that, following the feeding of a new printing plate and the establishment of the in-register position, the appropriate clamping rail can be closed in order to mount the plate. The control system which evaluates the signal, effects the closing of the rail and triggers the further processing of the changing operation is arranged outside of the cylinder. Thus, the signal from the electrically interrogable register pins is required to be lead out of the cylinder.
For leading the signal from the cylinder to the control unit, a rotary transmitter, such as a slip ring transmitter is commonly employed. However, the presence of a such a rotary transmitter assumes that there is appropriate installation space at one end of the cylinder. This installation space is not always available, in particular, in the case of plate cylinders having equipment for lateral and circumferential register adjustment. A slip ring transmitter also brings with it the disadvantage that soiling of the contacts may lead to signal interference, which in turn causes disruption to the plate-changing operation.
By way of further example, EP 0 551 976 discloses equipment for electrical register interrogation in which the signals from the electrically interrogable register pins are looped out via contact areas fitted to the plate cylinder. Corresponding contacts can by brought into contact with the contact areas fitted to the cylinder, these contacts being connected to a circuit which evaluates the signals. However, the disadvantage in this case is that appropriate space for the contact areas has to be provided on the cylinder and on the frame. Again, the soiling of the contact area enhances the risk of an erroneous signal transmission.
Still further, U.S. Pat. No. 5,320,041 (EP 0555782) discloses equipment for electrical register, in which signals from electrically interrogable register pins are looped out of the cylinder via a spit transformer. For this purpose, the cylinder has an appropriate transmission coil, towards which a further coil may be brought at a specific angular position of the cylinder. Because of the inductive signal transmission, the two coils need to be brought as close to one another as possible and thus it is necessary to provide an item of equipment which can be brought towards and away from the cylinder. Accordingly, this device suffers the aforementioned disadvantages.
Finally, DE 298 08 098 and DE 298 08 099 disclose equipment for register control in which the signals generated in the cylinder are led out by means of radiowaves and, respectively, optical beam transmission. The signal generated in each case may be encoded. Nevertheless, the transmission of radio or optical signals is not possible under specific machine conditions, for example, if other radiation sources or interference sources are present in the area of the cylinder.